reserve
  a for natural Number,
  k,l,m,n,k1,b,c,i for Nat,
  x,y,z,y1,y2 for object,
  X,Y for set,
  f,g for Function;
reserve p,q,r,s,t for FinSequence;
reserve D for set;

theorem Th40:
  for x being object holds p = <*x*> iff len p = 1 & p.1 = x
proof let x be object;
  len p = 1 iff dom p = Seg 1 by Def3;
  hence thesis by Def8;
end;
